The present invention relates, in general, to devices and methods for examining welds, and in particular, to a new and useful spot weld examination apparatus and method using EMATs.
The examination of spot welds using conventional piezoelectric transducers has been established as a viable method of determining spot weld quality. The use of these transducers requires a coupling fluid or gel to be able to generate and receive ultrasonic signals. The need for a liquid couplant makes it impractical to automate this test. A large volume of background material exists for this testing. An overview of this method of testing is given in the ASNT (American Society of Nondestructive Testing) Nondestructive Testing Handbook Vol. 7, pages 557-568 "Ultrasonic Testing of Spot Welds in Thin Gage Steel".
Other non ultrasonic methods of spot weld testing have been publicly disclosed. The following U.S. patents are known to relate to spot weld testing:
U.S. Pat. No. 3,726,130--Method of monitoring a welding operation;
U.S. Pat. No. 4,449,092--Acoustic wave spot welder adaptive control;
U.S. Pat. No. 4,887,025--Method and apparatus for non-destructive checking of spot welds between metal sheets;
U.S. Pat. No. 5,194,709--Method for checking a spot welded portion and spot welding machine; and
U.S. Pat. No. 5,399,827--Method and device for determining the temperature of a spot-welded joint and a method for assessing the quality of a spot welded joint.
In addition, a method of using Lamb waves to monitor spot welds is described in a paper "On-line Ultrasonic Lamb Wave Monitoring of Spot Welds" by S. I. Rokhlin, R. J. Mayhan and L. Adler, published in Materials Evaluation Vol. 43, No. 7, pages 879-883, ASNT, 1985.
At present, most spot welds used in manufacturing operations such as automotive chassis assembly are evaluated by destructively testing the welds in a statistical sampling of the assemblies. Due to economics, only a very small sampling of the assemblies are tested. If a welder develops a problem, a large number of assemblies with defective welds can be produced before the problem is detected. The manufacturer may have to scrap a large number of assemblies if a systematic problem in producing the welds is detected during destructive testing. In addition, bad welds can be produced at random, and for reasons other than welder problems. This results in assemblies that are of poorer quality that can fail prematurely in service. To compensate for the likelihood of the presence of bad welds in assemblies, the manufacturer may use a much larger number of welds than would be required if the welds were known to be good, increasing production costs. The majority of these welds receive little or no testing as to the quality of the weld.
An automated, reliable and cost effective method of nondestructively testing spot welds is needed to provide a measure of spot weld quality for each spot weld produced. The present invention satisfies that need.
An ultrasonic testing technique for nondestructive evaluation that does not need a fluid couplant has been developed within approximately the last twenty-five years. The technique depends upon electromagnetic acoustic interaction for elastic wave generation. Radio Frequency (RF) currents flowing in a flat coil placed close to a metal surface induce eddy currents in the metal surface. A static or quasi-static magnetic field is also applied to the metal surface. The eddy current flowing in the surface of the metal in the presence of the magnetic field experience a Lorentz force. This force then generates a mechanical disturbance, coupling to the atomic lattice by a scattering process. In electromagnetic acoustic generation, the electromagnetic conversion takes place directly within the eddy current skin depth. Therefore, no mechanical coupling to the body is needed. The metal surface is its own transducer. Reception takes place in a reciprocal manner.
If an elastic wave strikes the surface of the conductor in the presence of a magnetic field, induced currents are generated in the receiving coil. This is similar to the operation of an electric generator. These transducers are known as EMATs (ElectroMagnetic Acoustic Transducers). EMATs can produce a variety of wave modes including surface waves, longitudinal waves and horizontally polarized shear waves. The absence of a couplant makes it possible to design transducers that operate at elevated temperatures, can be rapidly scanned and can be easily automated. In addition, the operating characteristics of EMATs can be reproduced from one unit to another very easily.